Audio power amplifier

ABSTRACT

An audio power amplifier comprising a first stage voltage amplifier circuit, a second stage voltage amplifier circuit and a last stage power amplifier circuit. The first stage voltage amplifier circuit is of differential amplifier type including a pair of field effect transistors for voltage amplifying an audio frequency signal. The second stage voltage amplifier circuit includes a field effect transistor for voltage amplifying the output signal from the first stage voltage amplifier circuit. The second stage voltage amplifier circuit also serves as a drive stage. The last-stage power amplifier circuit includes a pair of field effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube for power amplifying the output signals of the second stage voltage amplifier circuit.

This application is a continuation-in-part of the now-abandoned United States application Ser. No. 409,586 filed Oct. 25, 1973, entitled AUDIO POWER AMPLIFIER in the name of TAKAO TAKEHARA.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to audio power amplifiers having a low frequency power amplifier stage constructed with power field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube (hereinafter referred to as triode characteristics), this construction permitting to simplify the circuit construction of a drive stage for driving the power amplifier circuit and also obtain production of high quality sounds with high fidelity.

The prior-art audio power amplifiers are roughly classed into two types, namely vacuum tube type and transistor type. The vacuum tube audio power amplifiers include those using pentodes and those using triodes.

Where pentodes are used, high gain and high efficiency can be obtained to obtain high power. In this case, however, problems are encountered in the non-linearility of the pentodes, giving rise to much odd harmonic distortion in the amplifier output, which is undesired from the acoustical standpoint. Also in this case, an output transformer must be used due to the high internal impedance of the pentode. This also contributes to the generation of distortions. Further, the required use of an output transformer is undesirable from the standpoint of the size and weight of the amplifier unit.

Amplifiers using triodes, on the other hand, provide superior lineality to that of the pentode amplifier, so that less negative feedback is required to reduce the distortion factor. Also, in this case the output transformer can be omitted since the internal impedance of the triode is low compared to the pentode. Further, even harmonic distortion constitutes the majority of the distortion component of the amplifier output. Since they have less adverse acoustical effects, sound of superior quality can be reproduced. However, the internal resistance of the triode is still considerably high although it is low compared to that of the pentode. Also, where the output transformer is omitted, it is necessary to provide negative feed-back to a great extent, giving rise to problems in the transient characteristics.

In addition, in the vacuum tube power amplifier using either pentodes or triodes, heater voltage is required, leading to increased power consumption and imposing restrictions upon the size reduction of the amplifier unit.

In the case of the transistor power amplifier, the reduction of size and weight as well as power consumption are possible to realize. However, the transistor has distortion characteristics similar to those of the pentode, mainly consisting of odd harmonic distortions. Therefore, the quality of the sound reproduced is inferior compared to the case of the triode vacuum tube power amplifier, so that the transistor power amplifier is unsuitable as a high-grade audio power amplifier for obtaining high fidelity sound reproduction. The audio power amplifier units described in the prior art are of significantly different construction as compared to the construction defined by the present invention, and particularly as regards the final stage. Thus, relevant prior art audio power amplifier units characteristically do not describe a final stage including power field effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube. Moreover such art does not describe a second stage voltage amplifier circuit having two outputs, reversed in phase and serving as a drive stage. Self-biased field-effect transistor amplifiers are known; however, such units do not include power field-effect transistors having the aforedescribed triode vacuum tube characteristics connected in the manner to be described in detail hereinafter.

SUMMARY OF THE INVENTION

In the light of the above aspects it is the primary object of the present invention to provide an audio power amplifier, which permits to obtain production of high quality sound with reduced odd harmonic distortion components, as well as permitting the reduction of the circuit construction and featuring small size and light weight constituting the merits of the transistor power amplifier.

Another object of the invention is to provide an audio power amplifier, in which a last-stage power amplifier is constructed with power field-effect transistors (hereinafter referred to as power EFT) having triode characteristics, thereby permitting to construct a previous voltage amplifier stage and a drive stage therefor as a single stage.

A further object of the invention is to provide an audio power amplifier, which comprises a pair of power FET's having triode characteristics and constituting a last-stage power amplifier circuit and an ordinary FET constituting a voltage amplifier circuit serving as drive stage for driving the power amplifier circuit.

A further object of the invention is to provide an audio power amplifier, which comprises voltage amplifier stages all constituted by ordinary FET's and a last-stage power amplifier circuit constituted by a pair of power FET's having triode characteristics.

A further object of the invention is to provide an audio power amplifier, which comprises a phase inversion and drive stage consisting of pair transistors in complementary symmetry connection and a last-stage power amplifier circuit consisting of pair power FET's having triode characteristics also in complementary symmetry connection.

A further object of the invention is to provide an audio power amplifier, which comprises a first-stage voltage amplifier circuit consisting of pair transistors in complementary symmetry connection, a drive stage also consiting of pair transistors in complementary symmetry connection and a last-stage power amplifier consisting of pair power FET's having triode characteristics also in complementary symmetry connection.

A yet further object of the invention is to provide an audio power amplifier, which comprises a drive stage and last-stage power amplifier circuit, each said stage consisting of pair FET's having triode characteristics. The foregoing objects are attained in accordance with the invention which in its broader aspects provides an audio power amplifier comprising a first-stage voltage amplifier circuit of differential amplifier type including a pair of field-effect transistors for voltage amplifying an audio frequency signal, a second-stage voltage amplifier circuit including a field-effect transistor for voltage amplifying the output signal from said first-stage voltage amplifier circuit said second-stage voltage amplifier circuit also serving as a drive stage, and a last-stage power amplifier circuit including a pair of field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube for power amplifying the output signals of said second-stage voltage amplifier circuit.

In a preferred embodiment of the invention, the second stage amplifier circuit includes an n-p-n transistor and p-n-p transistor connected in complementary symmetry form for phase inverting the output signal of the first stage amplifier circuit and the last-stage power amplifier circuit includes a pair of p-channel and n-channel power field effect transistors having the aforedescribed triode characteristics connected in complementary symmetry form for power amplifying the output signal from the n-p-n transistor being adapted to drive the p-channel power field-effect transistor and the output signal from the p-n-p transistor is adapted to drive the n-channel power field effect transistor.

In a further preferred embodiment, the first stage amplifier circuit includes an n-p-n transistor and a p-n-p transistor connected in complementary symmetry form for amplifying an audio frequency signal, the second stage amplifier circuit is connected to the output side of the first stage amplifier circuit, said second stage including a second n-p-n transistor and a second p-n-p transistor connected in complementary symmetry form and a last-stage power amplifier circuit including a p-channel power field-effect transistor and an n-channel power field-effect transistor connected in complementary symmetry form for amplifying the output signals of the second stage amplifier circuit and wherein the output signal from the second n-p-n transistor is adapted to drive the p-channel power field-effect transistor and the output signal from the second p-n-p transistor is adapted to drive the n-channel power field effect transistor.

According to yet another preferred embodiment of the invention, the second stage amplifier circuit includes a p-channel field-effect transistor and an n-channel field-effect transistor having the aforedescribed triode characteristics connected in complementary symmetry form for amplifying the output signal from the first stage amplifier circuit, the last stage power amplifier circuit includes a p-channel power field-effect transistor and an n-channel power field-effect transistor having the aforedescribed triode characteristics connected in complementary symmetry form for power amplifying the output signal from the p-channel field-effect transistor being adapted to drive the p-channel power field-effect transistor, the output signal from the n-channel field-effect transistor being adapted to drive the n-channel power field-effect transistor.

Other aspects and embodiments of the invention will become more apparent hereinafter as the description proceeds.

The features which are believed to be novel and characteristic of this invention are set forth with particularly in the appended claimed. The invention itself, however, together with the further objects and advantages thereof, will be best understood from the following description taken in conjunction with the accompanying drawings which illustrates, by way of example only, some preferred embodiments of the invention and throughout which like reference characters designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows drain voltage versus drain current characteristics of a power FET used for the audio power amplifier according to the invention.

FIG. 2 is a circuit diagram showing a first embodiment of the audio power amplifier according to the invention.

FIG. 3 is a circuit diagram showing a second embodiment of the audio power amplifier according to the invention.

FIG. 4 is a circuit diagram showing a third embodiment of the audio power amplifier according to the invention.

FIG. 5 is a circuit diagram showing a fourth embodiment of the audio power amplifier according to the invention.

FIG. 6 is a circuit diagram showing a fifth embodiment of the audio power amplifier according to the invention.

FIG. 7 is a circuit diagram showing a sixth embodiment of the audio power amplifier according to the invention.

FIG. 8 is a circuit diagram showing a seventh embodiment of the audio power amplifier according to the invention.

FIG. 9 is a circuit diagram illustrating an example of audio power amplifier of the invention.

FIG. 10 is a circuit diagram illustrating a further example of audio power amplifier of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows static drain voltage (V_(D)) versus drain current (I_(D)) characteristics of a field-effect transistor used for the audio power amplifier according to the invention, the characteristics being similar to static plate voltage versus plate current characteristics of a triode vacuum tube.

FIG. 2 shows an audio power amplifier embodying the invention. It is constructed by using field-effect transistors (hereinafter referred to as FET) having the afore-mentioned triode characteristics. In the Figure, reference symbol IN designates an input terminal, to which an audio frequency signal is coupled.

The input terminal IN is connected through a capacitor 2 C₁ to the base of a transistor Tr₁. The base of this transistor is also connected through a resistor R₁ to earth.

The transistor Tr₁ has its collector connected through a resistor R₂ and a parallel circuit of variable resistor VR₁ and capacitor C₂ to a terminal t₁, which is held at a potential -B₂.

The emitter of the transistor Tr₁ is directly connected to the emitter of another transistor Tr₂, the transistors Tr₁ and Tr₂ constituting a first-stage voltage amplifier circuit of differential amplifier type.

Both the transistors Tr₁ and Tr₂ have their emitter connected commonly through resistors R₃ and R₄ to a terminal t₂, which is held at a potential +B₁. The connection between the resistors R₃ and R₄ is connected through a capacitor C₃ to earth.

The transistor Tr₂ has its collector connected through a resistor R₅ and a parallel circuit of variable resistor VR₂ and capacitor C₄ to the terminal t₁, and its base is connected through a series circuit of resistor R₆ and capacitor C₅ to earth.

The collector of the transistors Tr₁ and Tr₂ is also connected directly to the base of respective transistors Tr₃ and Tr₄.

The transistor Tr₃ and Tr₄ have their emitters respectively connected to resistors R₇ and R₈, which are connected to each other at point P₁, which is in turn connected to a resistor R₉ connected through a parallel circuit of resistor R₁₀ and capacitor C₆ to earth and also through a resistor R₁₁ to the terminal t₁.

As is shown, the transistor Tr₃ is an n-p-n transistor, while the transistor Tr₄ is a p-n-p transistor. These transistors Tr₃ and Tr₄ constitute a second-stage voltage amplifier circuit serving both as phase inversion stage and drive stage.

The transistor Tr₃ has its collector connected through resistors R₁₂ and R₁₃ in series with each other to the terminal t₂, and the transistor Tr₄ has its collector connected through a resistor R₁₄ to the terminal t₁. The collector of both these transistors Tr₃ and Tr₄ is also connected to the gate of respective power field-effect transistors Tr₅ and Tr₆ (hereinafter referred to as power FET) having triode characteristics.

The power FET's Tr₅ and Tr₆ having triode characteristics constitute a last-stage power amplifier circuit the power Tr₅ having triode characteristics has its drain connected directly to the terminal t₂ and its source connected through a resistor R₁₅ to an output terminal T₁. The power FET Tr₆ having triode characteristics has its drain connected directly to the output terminal T₁ and its source connected through a resistors R₁₆ to a terminal t₃, which is held at a potential -B₁.

The output appearing between the output terminal T₁ and another output terminal T₂ which is grounded is taken out as output signal.

The output terminal T₁ is connected through a resistor R₁₇ to the base of the transistor Tr₂ for providing negative feedback to the main loop, and it is also connected through a capacitor C₇ to the connection point between the resistors R₁₂ and R₁₃ for forming a bootstrap loop (for positive feed-back).

The variable resistors VR₁ and VR₂ permit the adjustment of the d-c level of the output appearing between the output terminals T₁ and T₂. More particularly, they permit to vary the base bias potentials on the transistors Tr₃ and Tr₄ for adjusting the zero level of d-c component of the output signal.

The resistance of the series circuit of resistors R₁₂ and R₁₃ and that of the resistor R₁₄ are suitably selected to appropriately bias the gate of the power FET's Tr₅ and Tr₆ having triode characteristics. The potential -B₂ on the terminal t₁ is made more negative than the potential -B₁ on the terminal t₃ since the gate potential on the power FET Tr₆ having triode characteristics must be lower than the source potential.

With the audio power amplifier of the above construction according to the invention, an audio frequency signal coupled to the input terminal IN is first voltage amplified through the first-stage voltage amplifier circuit of transistors Tr₁ and Tr₂ constituting a differential amplifier.

The output signals appearing at the collector of the transistors Tr₁ and Tr₂ are respectively coupled to the base of the transistors Tr₃ and Tr₄ constituting the second-stage voltage amplifier circuit for further voltage amplification and phase inversion.

The phase inversion output signals appearing at the collector of the transistors Tr₃ and Tr₄ are respectively coupled as drive signal to the gate of the power FET's Tr₅ and Tr₆ having triode characteristics and constituting the last-stage power amplifier circuit. In this way, the audio frequency signal is power amplified.

The output of the power amplifier stage appearing at the output terminal T₁ is coupled as negative feedback signal through the resistor R₁₇ to the transistor Tr₂, and it is also used to provide bootstrap through the circuit of capacitor C₇ and resistor R₁₂ to the transistor Tr₃.

The power amplified audio frequency signal thus appearing between the output terminals T₁ and T₂ is less subject to odd harmonic distortions because the power FET's Tr₅ and Tr₆ in the power amplifier stage have characteristics similar to those of a triode vacuum tube.

While in the above embodiment a differential amplifier constituted by a pair of transistors Tr₁ and Tr₂ is used as the first-stage voltage amplifier circuit, this is by no means limitative, and a usual transistor amplifier may be used for this state as well. Also, this voltage amplifier circuit may consist of two differential amplifier stages.

FIG. 3 shows a second embodiment of the invention. In the Figure, reference symbol IN designates an input terminal, to which an audio frequency signal is added.

The input terminal IN is connected through a capacitor C₈ to the base of a transistor Tr₇. The base of this transistor is also connected to a movable terminal of a variable resistor VR₃.

The variable resistor VR₃ is parallel with a series diode circuit consisting of two series diodes D₁ and D₂. The diode D₁ has its anode side connected to earth and its cathode side connected to the anode side of the diode D₂, whose cathode side is connected through a resistor R₁₈ to the terminal t₁.

The emitter of the transistor Tr₇ is connected through resistors R₁₉ and R₂₀ in series with each other to terminal t₂ the connection between the resistors R₁₉ and R₂₀ being connected through a capacitor C₁₀ to earth.

The collector of the transistor Tr₇ is connected through a resistor R₂₁ to the terminal t₁. It is also connected directly to the gate of an ordinary field-effect transistor (hereinafter referred to as FET) Tr₈.

The transistor Tr₇ constitutes a first-stage voltage amplifier circuit, and the FET Tr₈ constitutes a second-stage voltage amplifier circuit. The FET Tr₈ serves both as phase inverter and as drive stage.

The drain of the FET Tr₈ is connected through resistors R₂₂ and R₂₃ in series with each other to the terminal t₂, and it is also connected directly to the gate of a power FET Tr₉ having triode characteristics.

The source of the FET Tr₈ is connected through a resistor R₂₄ to the terminal t₁, and it is also connected directly to the gate of a power FET Tr₁₀ having triode characteristics.

The power FET's Tr₉ and Tr₁₀ constitute a last-stage push-pull power amplifier circuit. The power FET Tr₉ has its drain connected to the terminal t₂ and its source connected through a resistor R₂₅ to output terminal T₁, and the power FET Tr₁₀ has its drain connected directly to the output terminal T₁ and its source connected through a resistor R₂₆ to terminal t₃.

The output of the power amplifier circuit of power FET's Tr₉ and Tr₁₀ appears as power amplified audio frequency signal between the output terminal T₁ and grounded output terminal T₂.

The output terminal T₁ is connected through a resistor R₂₇ to the emitter of the transistor Tr₇ of the first-stage voltage amplifier circuit for providing a negative feed-back to the main loop, and it is also connected through a capacitor C₉ to the connection point between the resistors R₂₂ and R₂₃ for forming a bootstrap circuit.

The variable resistor VR₃ is provided for presetting the base bias on the transistor Tr₇. By appropriately setting the base bias voltage the adjustment of the zero level of the d-c component of the output signal appearing between the output terminals T₁ and T₂ may be made. The diodes D₁ and D₂ are provided for stabilizing the voltage applied across the variable resistor VR₃.

With the audio power amplifier of the above construction according to the invention, an audio frequency signal coupled to the input terminal IN goes to the base of the transistor Tr₇ for voltage amplification.

The output signal appearing at the collector of the transistor Tr₇ is coupled to the gate of the FET Tr₈ for further voltage amplification.

An output appearing from the drain of the FET Tr₈ and in the opposite phase as the audio frequency input to the gate of this transistor is coupled to the gate of the power FET Tr₉ having triode characteristics. At the same time an output appearing from the source of the FET Tr₈ and in the same phase with the audio frequency input to the gate of this transistor is coupled to the gate of the power FET Tr₁₀. Thus, it will be seen that the FET Tr₈ also serves to drive the power FET's Tr₉ and Tr₁₀.

With the audio frequency inputs to the power FET's Tr₉ and Tr₁₀ constituting the power amplifier stage, the output from this stage appears at the output terminal T₁, and it is coupled as negative feed-back signal through the resistor R₂₇ to the transistor Tr₇ and also as bootstrap through the capacitor C₉ and resistor R₂₂ to the FET Tr₈.

The power amplified audio frequency signal thus appearing between the output terminals T₁ and T₂ is less subject to add harmonic distortions because the power FET's Tr₉ and Tr₁₀ constituting the power amplifier stage have characteristics similar to those of a triode vacuum tube.

FIG. 4 shows a third embodiment of the invention. In the Figure, reference symbol IN designates an input terminal, to which an audio frequency signal is coupled, and which is connected through a capacitor C₁₁ to the gate of a p-channel FET Tr₁₁. The gate of this transistor is also connected through a resistor R₂₉ to earth, and the input terminal IN is also connected through a resistor R₂₈ to earth.

The source of the FET Tr₁₁ is directly connected to the source of a similar p-channel FET Tr₁₂, and these sources are commonly connected through a resistor R₃₀ to earth, and also they are connected through a resistor R₃₁ to terminal t₁.

As is apparent from the Figure, these two FET's Tr₁₁ and Tr₁₂ form a differential amplifier, and they constitute a first stage voltage amplifier circuit. The FET Tr₁₁ has its drain connected through a variable resistor VR₄ to the terminal t₁, while the FET Tr₁₂ has its drain directly connected to the terminal t₁.

The FET Tr₁₂ has its gate connected through a series circuit of resistor R₃₂ and capacitor C₁₂ to earth. The drain of the FET Tr₁₁ is directly connected to the gate of an n-channel FET Tr₁₃.

The FET Tr₁₃ constitutes a second-stage voltage amplifier circuit, and it also serves as drive stage. Its source is connected through a resistor R₃₃ to the terminal t₁ and also connected directly to the gate of a power FET Tr₁₄ having triode characteristics to be described hereinafter and its drain is connected through a series circuit of resistors R₃₄ to R₃₇ to terminal t₂.

The connection between the resistors R₃₄ and R₃₅ is connected to the gate of an FET Tr₁₅. The FET Tr₁₅ is provided for temperature compensation. Its source is connected directly to the drain of the FET Tr₁₃, and its drain connected to the gate of another power FET Tr₁₆ having triode characteristics.

The two power FET Tr₁₄ and Tr₁₆ having triode characteristics constitute a last-stage power amplifier circuit. The power FET Tr₁₆ has its drain connected directly to the terminal t₂ and its source connected through a resistor R₃₈ to output terminal T₁. The power FET Tr₁₄ has its drain connected directly to the output terminal T₁ and its source connected through a resistor R₃₉ to terminal t₃.

The output of the power amplifier circuit consisting of the power FET's Tr₁₄ and Tr₁₆ appears as power amplified audio frequency signal between the output terminal T₁ and the grounded output terminal T₂. The output terminal T₁ is connected through a resistor R₄₀ to the gate of the FET Tr₁₂ for providing a negative feed-back, and it is also connected through a capacitor C₁₃ to the connection point between the resistors R₃₆ and R₃₇ for providing a bootstrap.

The variable resistor VR₄ is provided for pre-setting the gate bias voltage on the FET Tr₁₃. By appropriately setting this gate bias voltage the adjustment of the zero level of the d-c component of the output signal appearing between the output terminals T₁ and T₂ may be made.

With the audio power amplifier of the above construction according to the invention, an audio frequency signal coupled to the input terminal IN goes to the gate of the FET Tr₁₁ for voltage amplification through the differential amplifier state consisting of the FET's Tr₁₁ and Tr₁₂.

The output signal from the differential amplifier stage appearing at the source of the FET Tr₁₁ is coupled to the gate of the FET Tr₁₃ constituting the second-stage voltage amplifier circuit for further voltage amplification.

The output appearing from the source of the FET Tr₁₃ is coupled to the gate of the power FET Tr₁₄. At the same time, the output appearing at the drain of the FET Tr₁₃ is coupled through the resistors R₃₄ and R₃₅ to the gate of the other power FET Tr₁₆. It will be seen that the FET Tr₁₃ serves both to voltage amplify the output from the FET Tr₁₁ and drive the power FET's Tr₁₄ and Tr₁₆.

With the audio frequency inputs to the power FET's Tr₁₄ and Tr₁₆ constituting the power amplifier stage, the output from this stage appears at the output terminal T₁, and it is coupled as negative feed-back signal through the resistor R₄₀ to the FET Tr₁₂ and also as bootstrap through the capacitor C₁₃ and resistors R₃₆, R₃₅ and R₃₄ to the FET Tr₁₃.

The power amplified audio frequency signal thus appearing between the output terminals T₁ and T₂ is less subject to odd harmonic distortions because the power FET's Tr₁₆ constituting the power amplifier stage have characteristics similar to those of a triode vacuum tube.

FIG. 5 shows a fourth embodiment of the invention. In the Figure, reference symbol IN designates an input terminal, to which an audio frequency signal is coupled. The input terminal IN is connected to one of pair input terminals of an operational amplifier AMP, whose other input terminal is connected through a resistor R₄₁ to earth. The operational amplifier AMP also has pair power supply terminals, the negative power supply terminal being connected through a diode D₃ to earth and positive power supply terminal being connected through a diode D₄ to earth. The negative and positive power supply terminals of the operational amplifier AMP are also connected through respective resistors R₄₂ and R₄₃ to respective terminals t₁ and t₂.

Between the terminals t₁ and t₂, a series circuit consisting of resistors R₄₄ and R₄₅, a diode D₅ and resistor R₄₆ is connected. The connection point between resistor R₄₆ and diode D₅ is connected to an output terminal of the operational amplifier AMP.

The connection point between resistors R₄₄ and R₄₅ is connected to the base of an n-p-n transistor Tr₁₇, and the connection point between diode D₅ and resistor R₄₆ is connected to the base of a p-n-p transistor Tr₁₈. The emitters of both the transistors Tr₁₇ and Tr₁₈ are commonly connected through a resistor R₄₇ to earth.

The collector of the transistor Tr₁₇ is connected through a resistor R₅₂ to the terminal t₂, and the collector of the transistor Tr₁₈ is connected through a resistor R₄₈ to the terminal t₁. With the transistors Tr₁₇ and Tr₁₈ connected in complementary symmetry in this way, they form a second-stage amplifier circuit serving both as phase inversion stage and drive stage.

The collector of the transistor Tr₁₇ is connected to the gate of a p-channel power FET Tr₁₉ having triode characteristics, and the collector of the transistor Tr₁₈ is connected to the gate of an n-channel power FET Tr₂₀ also having triode characteristics.

The power FET Tr₂₀ has its drain connected to terminal t₂ and its source connected through a resistor R₄₉ to output terminal T₁. The power FET Tr₁₉ has its source connected through a resistor R₅₀ to the output terminal T₁ and its drain connected to terminal t₁. The two power FET's Tr₁₉ and Tr₂₀ having triode characteristics and connected in complementary symmetry in this way form a last-stage power amplifier circuit.

The output of this power amplifier circuit appears between the output terminal T₁ and grounded output terminal T₂. The output terminal T₁ is connected through a resistor R₅₁ to the aforementioned other input terminal of the operational amplifier AMP for negative feed-back.

With the audio power amplifier of the above construction an audio frequency signal coupled to the input terminal IN is first amplified through the operational amplifier AMP.

The output from the operational amplifier AMP is coupled through the diode D₅ and resistor R₄₅ to the base of the transistor Tr₁₇, and also directly to the base of the transistor Tr₁₈, for further voltage amplification and phase inversion.

The phase inversion output signals appearing at the collector of the transistors Tr₁₇ and Tr₁₈ are respectively coupled as drive signal to the gate of the power FET's Tr₁₉ and Tr₂₀. In this way, the audio frequency signal is power amplified.

The output signal of the power amplifier stage consisting of the power FET's Tr₁₉ and Tr₂₀ appears at the output terminal T₁, and it is coupled as negative feed-back signal through the resistor R₅₁ to the operational amplifier AMP.

The power amplified audio frequency signal thus appearing between the output terminals T₁ and T₂ is less subject to odd harmonic distortions because the power FET's Tr₁₉ and Tr₂₀ constituting the power amplifier stage have characteristics similar to those of a triode vacuum tube. The reduction of the odd harmonic distortions is further enhanced with the complementary symmetry connection of the second-stage and last-stage amplifier circuits, so that sound of very high quality can be reproduced from a loud-speaker connected between the output terminals T₁ and T₂.

FIG. 6 shows a modification of the preceding embodiment of FIG. 5. In the Figure, like parts as those in FIG. 5 are designated by like reference symbols and are not described in any further.

In this embodiment, a differential amplifier consisting of pair transistors Tr₂₁ and Tr₂₂ is used as a first-stage amplifier circuit in place of the operational amplifier AMP in the embodiment of FIG. 5. The base of the transistor Tr₂₁ is connected through a capacitor 14 to input terminal IN, and it is also connected through a resistor R₅₃ to earth.

The transistors Tr₂₁ and Tr₂₂ have their emitters commonly connected through resistors R₅₄ and R₄₃ to terminal t₂, the connection point between the resistors R₅₄ and R₄₃ being connected through a capacitor C₁₅ to earth.

The transistor Tr₂₁ has its collector connected through a resistor R₅₅ to terminal t₁, and the transistor Tr₂₂ has its collector connected directly to the terminal t₁ and its base connected through a series circuit of resistor R₅₆ and capacitor C₁₆ to earth.

The collector of the transistor Tr₂₁ is also connected to the base of a transistor Tr₂₃. This transistor Tr₂₃ is provided for phase inversion. Its emitter is connected through a resistor R₅₇ to the terminal t₁, and its collector is connected through a series circuit of resistor R₄₅, diodes D₆ and D₇ and resistor R₄₄ to terminal t₂. The diodes D₆ and D₇ are provided for temperature compensation.

The collector of the transistor Tr₂₃ is also connected to the base transistor Tr₁₈, and the connection point between resistor R₄₄ and diode D₆ is connected to the base of a transistor Tr₁₇.

With the above construction, an audio frequency signal coupled to the input terminal IN is voltage amplified through the first-stage amplifier circuit consisting of the transistors Tr₂₁ and Tr₂₂.

The output of this stage appearing from the collector of the transistor Tr₂₁ is coupled to the base of the transistor Tr₂₃ for phase inversion.

The phase inverted output signal appearing from the collector of the transistor Tr₂₃ is coupled through resistor R₄₅ and diodes D₇ and D₆ to the base of the transistor Tr₁₇, while it is also coupled directly to the base of the transistor Tr₁₈.

The phase inverted signal thus coupled to the bases of the transistors Tr₁₇ and Tr₁₈ are amplified in the same way as is described before in connection with the previous embodiment of FIG. 5, so that the power amplified audio frequency signal from the power amplifier circuit of the power FET's Tr₁₉ and Tr₂₀ having triode characteristics appears at the output terminal T₁.

FIG. 7 shows a sixth embodiment of the invention. In the Figure, reference symbol IN designates an input terminal, to which an audio frequency signal is coupled. The input terminal IN is connected through a capacitor C₁₇ to the base of a transistor Tr₁₄ and also through a capacitor C₁₈ to the base of a transistor Tr₂₅.

Between the bases of both the transistors Tr₂₄ and Tr₂₅ is connected series circuit consisting of resistor R₅₉, capacitor C₁₉ and resistor R₆₀. The base of the transistor Tr₂₄ is also connected through resistors R₆₁ and R₆₃ in series with each other to terminal t₂, the connection point between the resistors R₆₁ and R₆₃ being connected through a capacitor C₂₀ to earth. The base of the transistor Tr₂₅ is connected through series resistors R₆₂ and R₆₄ to terminal t₁, the connection point between the resistors R₆₂ and R₆₄ being connected through a capacitor C₂₁ to earth.

The transistors Tr₂₄ and Tr₂₅ have their emitters commonly connected through a resistor R₆₇ to earth.

The collector of the transistor Tr₂₄ is connected through a resistor R₆₅ to the juncture between the resistors R₆₁ and R₆₃, and the collector of the transistor Tr₂₅ is connected through a resistor R₆₆ to the juncture between the resistors R₆₂ and R₆₄. The n-p-n transistor Tr₂₄ and p-n-p transistor Tr₂₅ connected in complementary symmetry in this way constitute a first-stage voltage amplifier circuit.

The collector of the transistor Tr₂₄ is also connected directly to the base of a transistor Tr₂₆, and the collector of the transistor T₂₅ is also connected directly to the base of the transistor Tr₂₇.

The collector of the transistor Tr₂₆ is connected through a resistor R₆₈ to the terminal t₁, and the collector of the transistor Tr₂₇ is connected through a resistor R₆₉ to the terminal t₂.

The emitter of the transistor Tr₂₆ is connected through resistors R₇₀ and R₇₁ in series with each other to the terminal t₂, the connection point between the resistors R₇₀ and R₇₁ being connected through a capacitor C₂₂ to earth.

Likewise, the emitter of the transistor Tr₂₇ is connected through resistors R₇₂ and R₇₃ in series with each other to the terminal t₁, the connection point between the resistors R₇₂ and R₇₃ being connected through a capacitor C₂₃ to earth.

The p-n-p transistor Tr₂₆ and n-p-n transistor Tr₂₇ connected in complementary symmetry in this way constitute a second-stage amplifier circuit serving both as phase inversion stage and drive stage.

The collector of the transistor Tr₂₆ is also connected to the gate of an n-channel power FET Tr₂₈ having triode characteristics, and the collector of the transistor Tr₂₇ is also connected to the gate of a p-channel power FET Tr₂₉ having triode characteristics.

The power FET Tr₂₈ has its drain connected to the terminal t₂ and its source connected through a resistor R₇₄ to output terminal T₁, and the power FET Tr₂₉ has its source connected through a resistor R₇₅ to the output terminal T₁ and its drain connected to the terminal t₁. The power FET's Tr₂₈ and Tr₂₉ connected in complementary symmetry in this way and both having triode characteristics constitute a last-stage push-pull power amplifier circuit.

With the audio power amplifier of the above construction, an audio frequency signal coupled to the input terminal IN goes through the capacitors C₁₇ and C₁₈ to the base of the respective transistors Tr₂₄ and Tr₂₅ constituting the first voltage amplifier stage for voltage amplification.

The output signals appearing from the collector of the respective transistors Tr₂₄ and Tr₂₅ is coupled to the base of the respective transistors Tr₂₆ and Tr₂₇ for further voltage amplification and phase inversion.

The phase inversion output signals appearing from the collector of the transistors Tr₂₆ and Tr₂₇ is coupled as drive signal to the gate of the respective power FET's Tr₂₈ and Tr₂₉.

The output of the power amplifier stage consisting of the power FET's Tr₂₈ and Tr₂₉ is taken out as output signal from the outpt terminal T₁.

This output is less subject to odd harmonic distortions because the power FET's Tr₂₈ and Tr₂₉ constituting the power amplifier stage have characteristics similar to those of a triode vacuum tube. The reduction of the odd harmonic distortions is further enhanced with the complementary symmetry connection of all the first to last amplifier stages, so that sound of very high quality can be reproduced from a loudspeaker connected between the output terminals T₁ and T₂.

FIG. 8 shows a seventh embodiment of the invention. In the Figure, reference numeral IN designates an input terminal, to which an audio frequency signal is coupled, and which is connected through a capacitor C₂₄ to the base of a transistor Tr₃₀. The input terminal IN and the base of the transistor Tr₃₀ are also connected through respective resistors R₇₆ and R₇₇ to earth.

The emitter of the transistor Tr₃₀ is directly connected to the emitter of a transistor Tr₃₁, and the common emitter connection is connected through a resistor R₇₈ to a terminal t₁, which is held at a potential +B₃.

As is shown, the transistor Tr₃₀ and Tr₃₁ form a differential amplifier constituting a first-stage voltage amplifier circuit.

The base of the transistor Tr₃₁ is connected through resistor R₇₉ and capacitor C₂₅ in series with each other to earth, and it is also connected through a resistor 80 to output terminal T₁ for negative feed-back.

The transistor Tr₃₀ has its collector connected through a resistor R₈₁ to a terminal t₂, which is held at a potential -B₃. The transistor Tr₃₁ has its collector directly connected to the terminal t₂.

The collector of the transistor Tr₃₀ is also connected to the base of a transistor Tr₃₂ which is provided for amplification. The transistor Tr₃₂ has its emitter connected to the terminal t₂ and its collector connected to the gate of a power FET Tr₃₃ having triode characteristics and described hereinafter. The collector of the transistor Tr₃₂ is also connected through a series circuit of resistors R₈₂ to R₈₇ to the terminal t₁. The connection point between the resistors R₈₅ and R₈₆ is connected to the gate of an FET Tr₃₅ having triode characteristics. The connection point between the resistors R₈₃ and R₈₄ is connected to the gate of an FET Tr₃₄ which is provided for temperature compensation. The FET Tr₃₄ has its source connected to the juncture between the resistors R₈₂ and R₈₃ and its drain connected to the juncture between the resistors R₈₄ and R₈₅. The connection point between the resistors R₈₆ and R₈₇ is connected through a capacitor C₂₆ to output terminal T₁.

The FET Tr₃₃ is of an n-channel type, and the FET Tr₃₅ is of a p-channel type. The FET Tr₃₃ has its drain connected to the output terminal T₁ and its source connected through a resistor R₈₉ to a terminal t₄, and the FET Tr₃₅ has its drain connected to the output terminal T₁ and its source connected through a resistor R₈₈ to a terminal t₃. The terminals t₃ and t₄ are held at respective potentials +B₂ and -B₂. The power FET's Tr₃₃ and Tr₃₅ connected in complementary symmetry in this way constitute a drive stage.

The source of the FET Tr₃₃ is connected to the gate of a power FET Tr₃₆ having triode characteristics. The power FET Tr₃₆ has its drain connected through a resistor R₉₁ to the output terminal T₁ and its source connected to a terminal t₆. The source of the FET Tr₃₅ is connected to the gate of a power FET Tr₃₇ having triode characteristics. The power FET Tr₃₇ has its source connected to a terminal t₅ and its drain connected through a resistor R₉₀ to the output terminal T₁. The terminals t₅ and t₆ are held at respective potentials +B₁ and -B₁. The two power FET's Tr₃₆ and Tr₃₇ having triode characteristics and connected in complementary symmetry in this way constitute a final-stage power amplifier circuit, whose output appears between output terminals T₁ and T₂. As is apparent from the Figure, the power FET Tr₃₆ is of an n-channel type, and the power FET Tr₃₇ is of a p-channel type. The power FET Tr₃₆ is adapted to be driven by the output signal from the n-channel FET Tr₃₃ having triode characteristics, and the power FET Tr₃₇ is adapted to be driven by the output signal from the p-channel FET Tr₃₅ having triode characteristics.

With the audio power amplifier of the above construction, an audio frequency signal coupled to the input terminal IN goes through the capacitor C₂₄ to the base of the transistor Tr₃₀ for voltage amplification through the first-stage voltage amplifier circuit of the transistors Tr₃₀ and Tr₃₁.

The output of this stage appearing at the collector of the transistor Tr₃₀ is coupled to the base of the transistor Tr₃₂ for further amplification thereby.

The output appearing at the collector of the transistor Tr₃₂ is coupled directly to the gate of the drive stage FET Tr₃₃ having triode characteristics and also coupled through the resistors R₈₂ and R₈₅ to the gate of the other drive stage FET Tr₃₅ also having triode characteristics.

In this way, the output signal from the transistor Tr₃₂ is amplified and phase inverted through the FET's Tr₃₃ and Tr₃₅ having triode characteristics, so that phase inversion output signals appear at the source of the FET Tr₃₃ and source of the FET Tr₃₅.

These output signals are coupled as drive signal to the gate of the respective power FET's Tr₃₆ and Tr₃₇ constituting the last-stage power amplifier circuit for power amplification of the preceding stage output.

The output of the power amplifier circuit of the power FET's Tr₃₆ and Tr₃₇ is taken out as output signal from the output terminal T₁.

The power amplified audio frequency signal thus obtained from the power amplifier circuit is less subject to odd harmonic distortions because the drive stage and output stage each consist of pair FET's having triode characteristics and connected in complementary symmetry form, so that sound of very high quality can be produced from a loudspeaker connected between the output terminals T₁ and T₂.

FIG. 9 shows an example of audio power amplifier in this invention, a differential amplifier circuit 10 amplies audio frequency signal and the differential amplifier circuit 10 is composed of two N-channel FET's Tr₃₈ and Tr₃₉.

A constant-current circuit 11 constantly controlling the electric current supplied to the said differential amplifier circuit 10 consists of transistors Tr₄₀.

A circuit 12 determines the bias of FET's Tr₃₈ Tr₃₉ of the said differential amplifier circuit 10 and of transistor Tr₄₀ of the said constant-current circuit 11.

A shunt regulated push-pull circuit 13 comprising drive stage is composed of four N-channel FET's Tr₄₁ -Tr₄₄ and its input side is connected to the output side of said differentiate amplifier circuit 10.

A constant-current circuit 14 constantly controlling the electric current supplied to said shunt regulated push-pull circuit 13 consists of transistor Tr₄₈.

A parallel push-pull circuit 15 comprising power amplifier stage is consists of four power FET's Tr₄₆ -Tr₄₉ having triode characteristics.

A circuit 16 determining gate bias of respective power FET's Tr₄₆ -Tr₄₉ is connected to the point between the input side of said parallel push-pull circuit 15 and the output side of said shunt regulated push-pull circuit 13. In the said power amplifier stage, the connection point P1 between the source of power FET Tr₄₆ and the drain of power FET Tr₄₈ is connected to the connection point P2 between the source of power FET Tr₄₇ and the drain of power FET Tr₄₉. Either of these connection points P₁ or P₂ is connected to the output terminal T₁.

FIG. 10 shows an example of modifications of the power amplifier stage and the circuit 16 that determines the bias shown in FIG. 9. This power amplifier circuit 15 connecting four power FET's Tr₄₆ -Tr₄₉ having triode characteristics in bridge fashion has a output terminal T₁ at the connection point P₁ connecting the source S of power FET Tr₄₆ and the drain D of power FET Tr₄₈ and (has) a terminal T₂ at the connection point P₂ connecting the source S of power FET Tr₄₇ and the drain of power FET Tr₄₉, and between these terminals T₁ and T₂ a loudspeaker 17 is connected. Thus power amplifier circuit 15 constitutes balanced transformer less.

Further, the gate G of power FET's Tr₄₆ and Tr₄₉ in the paid power amplifier circuit is connected between the source of FET Tr₄₁ and the drain of FET Tr₄₂ in said shunt regulated push-pull circuit 13.

The gate G of power FET's Tr₄₇ and Tr₄₈ is connected between the source of FET Tr₄₃ and the drain of FET Tr₄₄ in the said shunt regulated push-pull circuit 13, and the respective gate bias of respective power FET's Tr₄₆ -Tr₄₉ is constituted respectively by variable resisters VR₅ -VR₈ that are parallel with each other.

A latitude of modification, substitution and change is intended in the foregoing disclosure, and in some instances, some features of the present invention may be employed without a corresponding use of other features. 

I claim:
 1. An audio power amplifier comprising a first-stage voltage amplifier circuit of differential amplifier type including a pair of field-effect transistors for voltage amplifying an audio frequency signal, a second-stage voltage amplifier circuit including a field effect transistor for voltage amplifying the output signal from said first-stage voltage amplifier circuit, said second-stage voltage amplifier circuit also serving as a drive stage, and a last-stage power amplifier circuit including a pair of field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube for power amplifying the output signals of said second-stage voltage amplifier circuit.
 2. An audio power amplifier comprising:a first-stage amplifier circuit for amplifying an audio frequency signal, a second-stage amplifier circuit including an n-p-n transistor and a p-n-p transistor, said transistors being connected in complementary symmetry form for phase inverting the output signal of said first-stage amplifier circuit, said second-stage amplifier circuit also serving as a drive stage, and a last-stage power amplifier circuit including a pair of p-channel and n-channel power field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube and connected in complementary symmetry form for power amplifying output signals from said second-stage amplifier circuit, the output signal from said second-stage n-p-n transistor being connected to drive said p-channel power field-effect transistor having triode characteristics, the output signal from said p-n-p transistor being connected to drive said n-channel power field-effect transistor having triode characteristics.
 3. The audio power amplifier according to claim 2, wherein said first-stage amplifier circuit for amplifying an audio frequency signal consists of an operational amplifier.
 4. The audio power amplifier according to claim 2, wherein said first-stage amplifier circuit for amplifying an audio frequency signal is a differential amplifier including a pair of transistors.
 5. An audio power amplifier comprising a first-stage amplifier circuit including an n-p-n transistor and p-n-p transistor, said transistors being connected in complementary symmetry form for amplifying an audio frequency signal, a second-stage amplifier circuit connected to the output side of said first-stage amplifier circuit including a second n-p-n transistor and a second p-n-p transistor, said second transistors being connected in complementary symmetry form, said second-stage amplifier circuit also serving as a drive stage, and a last-stage power amplifier circuit including a p-channel power field-effect transistor and a n-channel power field-effect transistor, said field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube and being connected in complementary symmetry form for amplifying output signals of said second-stage amplifier circuit, the output signal from said second n-p-n transistor being connected to drive said p-channel power field-effect transistor, the output signal from said second p-n-p transistor being connected to drive said n-channel power field-effect transistor.
 6. An audio power amplifier comprising:a first-stage amplifier circuit for amplifying an audio frequency signal, a second-stage amplifier circuit including a p-channel field-effect transistor and an n-channel field-effect transistor, said field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube and being connected in complementary symmetry form for amplifying the output signal from said first-stage amplifier circuit, and a last-stage power amplifier circuit including a p-channel power field-effect transistor and an n-channel power field-effect transistor both having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube and being connected in complementary symmetry form for power amplifying output signals from said second-stage amplifier circuit, the output signal from said second-stage p-channel field-effect transistor being connected to drive only said p-channel power field-effect transistor, and the output signal from said second-stage n-channel field-effect transistor being connected to drive only said n-channel power field-effect transistor. 